1. Field of the Invention
The present invention generally relates to a circuit substrate structure on which to mount IC chips and the like and, more particularly, to a circuit substrate structure having multiple wiring layers.
The present invention also relates to a circuit apparatus in which IC chips (circuit elements) and the like are mounted on a circuit substrate structure provided with multiple wiring layers.
2. Description of the Related Art
It has become common to provide a cell phone with a variety of additional features other than communication functions. A multi-featured cell phone is valued highly in the market and can establish itself as a highly popular model. Particularly in recent years, cell phones with a built-in FM tuner for receiving radio broadcast are gaining popularity. Manufacturers are making efforts to reduce the size of an FM tuner.
An FM tuner requires two spiral coil patterns in an oscillation circuit. There is proposed in the related art a high-frequency oscillation circuit in which a maximum oscillation frequency is improved by using a MOS transistor in which a bulk potential is controllable, instead of using a diode (see, for example, patent document 1). There is a also proposed a circuit substrate structure provided with an LC circuit formed such that the peripheries of multiple coil patterns face opposite to each other across a dielectric (see, for example, patent document 2).
[Patent Document No. 1]
    JP 2001-332931 A[Patent Document No. 2]    JP 2004-87524 A
In forming two coil patterns on a wiring layer and fabricating a circuit substrate structure in which an end of a coil pattern is connected to an end of the other coil pattern, a via is formed in a dielectric layer underlying the wiring layer to extend to another wiring layer underlying the dielectric layer. A bridge electrically connects between vias. Due to electromagnetic field produced around the bridges and the coil patterns, the operating performance of a nearby circuit may be adversely affected.
For example, when a circuit substrate structure is mounted on aboard such that the side provided with a bridge faces downward, electromagnetic interference with wirings formed on the surface of the board or in the neighborhood thereof causes variation in the operating performance of the circuit substrate structure. In this case, wirings formed on the board are also affected by electromagnetic interference from the bridges and the coil patterns in the circuit substrate structure. This may create difficulty in fixing the oscillation frequency of an oscillation circuit and prevent appropriate tuning.
One conceivable approach to address this situation is to suppress electromagnetic leakage from the bridges and the coil patterns by coating the entirety of a surface sealing the bridges with a conductive layer (ground). By forming a conductive layer on the entirety of the sealing surface, the number of steps is increased so that fabrication cost is increased accordingly. Another problem is that, by forming a conductive layer, the overall thickness of the circuit substrate structure is increased so that the requirement for miniaturization is not met.
The above-mentioned approach is capable of suppressing electromagnetic leakage to the board on which to mount the circuit substrate structure provided with the coil patterns but cannot successfully suppress electromagnetic leakage from the coil patterns in a direction away from the board. Depending on the condition in which the circuit substrate structure is built in an electronic appliance, the leakage may cause similar malfunction by creating electromagnetic interference with wirings formed in another board located in close proximity to the circuit substrate structure. One possible countermeasure to address this problem is to coat the entirety of a surface, which is opposite to the board on which to mount the circuit substrate structure provided with the coil patterns, with a conductive layer (ground), so as to suppress electromagnetic leakage from the coil patterns.
However, by forming a conductive layer on the entirety of the surfaces of the circuit substrate structure provided with the coil patterns, the number of steps is increased so that fabrication cost is increased accordingly. By forming a conductive layer on both surfaces of the circuit substrate structure, the number of wiring layers is increased so that the overall thickness of the circuit substrate structure is increased and the requirement for miniaturization is not met.